Geckos are one of nature's most agile and power-efficient climbers due to their strong, highly repeatable, high speed, and controllable attachment and detachment capabilities on a wide range of smooth and slightly rough surfaces. Such capabilities are a result of angled and hierarchical micro and/or nanoscale fibrillar structures on their feet, which have saucer shaped tip endings. These micro/nanostructures can exhibit repeatable adhesive strengths up to 200 kPa on smooth and rigid surfaces such as glass. The attachment strength of gecko foot-hairs has been shown to be rooted in intermolecular forces such as van der Waals forces, which exist between all surfaces and is fairly insensitive to surface chemistry. This generic attachment principle enables the animal to climb on a wide range of surface materials. The importance of geometry, size, material type, and surface physics of these biological foot-hairs to their adhesion strength, rather than their surface chemistry, leads these biological adhesives to be called structured adhesives. Many researchers have proposed methods to design and fabricate such synthetic micro/nanostructured adhesives inspired by gecko foot-hairs.
In addition to high attachment strength, biological micro/nanofibrillar structures exhibit highly controllable adhesion. The controlled adhesion and shear strength of gecko's angled fibrillar structures is dependent on mechanical deformations induced by vertical and lateral loading of its feet, which can actively control the contact area between the structures and the substrate. It has been demonstrated that gecko foot-hairs have a friction ratio of around 5 to 1 comparing the with to against hair tilt directions.
Synthetic structured adhesives have been designed in an attempt to mimic the strength and controllability of these biological foot-hairs. It has been shown that when stiff polymer microfiber arrays are angled they exhibit anisotropic behavior of shear strength with a ratio of 45 to 1 between dragging resistance in with and against fiber tilt directions. However, in both of these vertical and angled cases, the microfibers had low adhesive strength. Researchers have used multi-wall carbon nanotubes (MWCNT) to create a structured surface with even smaller features that exhibit adhesive strength of 100 kPa and shear strength of 80 kPa. Similarly, embedding MWCNT arrays in polymer backing showed enhanced friction, but these MWCNT surfaces lacked controllable adhesion.
In the study with results closest to the strength and controllability of biological foot-hairs, researchers have developed elastomer, angled polymer fibers with angled mushroom shaped tip endings which demonstrated interfacial shear pressures of 100 kPa and adhesion pressure of 50 kPa. These structures exhibited controlled shear and adhesion strength: with-to-against friction ratios of around 5 to 1 and adhesion ratios of 35 to 1. Subsequently, surface treatments have been used to enhance adhesion of polymer microfibers in air and under water. In a different approach to adhesion control, thermal control has been used on shape memory polymer fiber arrays.
The aforementioned preload- and shear-controlled adhesion and friction properties could be one of the major reasons why biological gecko foot-hairs can shed dirt particles in dry conditions. Researchers have demonstrated that dirt microparticles much larger than the fiber tip diameter could be shed from the gecko's foot after it is attached to and detached from a clean glass substrate in many cycles, a process termed contact self-cleaning. Such contact self-cleaning property has been also been shown in synthetic polymer fiber adhesives by shear loading. These studies suggest that micro/nanostructures could also be used for pick-and-place manipulation of micro or macroscale parts since they enable controlled attachment (pick) and detachment (release). Therefore, microstructured adhesives inspired by these biological structures have recently been used for manipulation at the micro and macroscale.
Researchers have presented elastomer micropyramidal structures as adhesion controlled micromanipulators. These microstructures used vertical compression induced contact area control such that there was a relatively large contact area when sufficiently large compressive loads buckled the microstructures. If pulled away quickly, the planar part was picked up with a high pull-off force because rate-dependent effects enhanced the adhesion strength further. After the part was picked, the buckled elastic structures reverted to their original shapes. This shape recovery significantly reduced the contact area, and thus, adhesion, between the pyramid structures and the part and enabled easy part release. The maximum ratio of pick to release adhesive forces was 1000 to 1. But, this manipulator had small holding forces after lifting the part from the donor substrate. Though the holding force was not measured directly, the observed contact area while holding a part was three orders of magnitude less than while picking up a part, indicating a holding force less than 1 nanonewtons, which could be a problem for heavy parts or for mechanical disturbances during transfer of the parts. Researchers have addressed this limitation by removing the micropyramidal structures of the manipulator and used shear displacement control to reduce attachment strength, at the cost of a reduced pick to release force ratio, but the force ratio was not presented. Researchers has utilized angled nanofibers with high shear strength to transfer thin film transistor (TFT) displays under vacuum or air as a macroscale manipulation demonstration. However, micron scale part manipulation was not demonstrated and the nanofiber array required a constant application of shear force for strong adhesion.